Sea serpents have been part of world mythology for thousands of years. People have reported sightings of sea serpents, but they have never been proven to exist. Sea serpents are said to range from 33 – 230 feet and have a horse-like head and fins down their backs. It is thought they move in an up-and-down motion. Viking mythology featured a sea serpent called Jormungandr; was so long it circled the whole world.